RF modulator and switch having high antenna isolation

CROSS REFERENCE TO RELATED APPLICATIONS

Not Applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

BACKGROUND OF THE INVENTION

The present invention relates in general to RF modulators used for coupling audio signals to an audio reproduction system via an RF antenna input, and, more specifically, to an RF modulator system having high isolation of the receiving antenna input when the modulator is operating.

RF modulators are used for coupling an audio signal into an audio system via its radio tuner when an auxiliary input to the audio system is not available. In an automotive audio system, for example, it may be desired to install a multimedia accessory such as a DVD player, a video cassette player, or a CD disk changer so that audio signals from the accessory can be heard over the speakers of the vehicle audio system. A head unit for such a system may often lack an available auxiliary input for receiving an accessory audio signal. The modulator modulates the accessory audio signal according to the modulation scheme used by a radio reception band of the radio tuner (e.g., FM) so that an auxiliary input in the audio system becomes unnecessary.

The modulated accessory signal may be coupled to the input of the radio tuner by short-range wireless broadcasting (i.e., through the air) or by a direct hardwired connection to the radio receiver's antenna input. When a hardwired connection is employed, a relay is typically used so that only the antenna or the modulator are connected to the antenna input at one time. The relay separates the radio frequency output of the modulator from the vehicle receiving antenna to help prevent the broadcast of the modulated accessory signal from the antenna when the modulator is operating. Disconnecting the antenna from the antenna input of the radio receiver while the modulator is operating also reduces interference from radio stations broadcasting on the same or nearby frequencies as the modulator.

A typical RF modulator may operate using frequency modulation at the lower end of the FM broadcast band. The frequency may be selectable between several standard FM channels (e.g., 87.9 MHz and 88.1 MHz) to avoid using a channel that is also being used by a local radio station transmitter. Even when a direct connection of the RF modulator to the antenna input is made via a relay, however, it has continued to be desirable or even necessary to use a modulator frequency different from any local broadcast stations because the broadcast signal has continued to be picked up at the antenna input thereby causing interference even without a direct antenna connection.

Due to the size and construction of the mechanical relays or micro-relays used for switching the antenna or the RF modulator to the antenna input of the radio receiver, incomplete isolation between all the terminals of the relay is achieved. Specifically, leakage capacitance and leakage inductance between terminals of the relay allows 1) radio broadcast signals to be coupled from the antenna line to the radio's antenna input thereby causing interference, and 2) modulated accessory signals to be coupled from the modulator to the antenna line thereby causing unintended RF emissions.

SUMMARY OF THE INVENTION

The present invention has the advantage of providing extremely high isolation between the reception antenna and both the modulator and the antenna input of the audio system, whereby interference and RF emissions are greatly reduced and the need for selectable frequencies in the modulator and the associated costs are eliminated.

In one aspect of the invention, RF switch is provided for selectably coupling an internally generated RF signal or an RF signal from an antenna to a broadcast radio receiver. An antenna RF input receives the vehicle antenna RF signal in response to a radiated RF broadcast in a broadcast band. A resonant network has a first node coupled to the antenna RF input and has a resonant frequency corresponding to a selected carrier frequency in the broadcast band. A first relay has a first output selectably coupled to either the antenna RF input or a second node of the resonant network. A second relay has a second output selectably coupled to either the local RF input or the first output of the first relay. When the second relay selects the internally generated RF input then the first relay selects the second node of the resonant network and when the second relay selects the first output of the first relay then the first relay selects the antenna RF input.

BRIEF DESCRIPTION OF THE DRAWINGS

<figref idref="DRAWINGS">FIG. 1</figref> is a block diagram showing a prior art hardwired RF modulator wherein normal radio reception is selected.

<figref idref="DRAWINGS">FIG. 2</figref> is a block diagram showing the RF modulator of <figref idref="DRAWINGS">FIG. 1</figref> wherein a modulated accessory signal is selected.

<figref idref="DRAWINGS">FIG. 3</figref> is a block diagram showing a preferred embodiment of an RF modulator/RF switch according to the present invention.

<figref idref="DRAWINGS">FIG. 4</figref> is a schematic diagram showing the modulator/switch in greater detail and configured to select normal radio reception.

<figref idref="DRAWINGS">FIG. 5</figref> is a schematic diagram showing the modulator/switch in greater detail and configured to select the output of the modulator.

<figref idref="DRAWINGS">FIG. 6</figref> is a schematic diagram showing an alternative embodiment of a resonant isolator of the present invention.

<figref idref="DRAWINGS">FIG. 7</figref> is a plot showing transfer characteristics of two different embodiments of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to <figref idref="DRAWINGS">FIG. 1</figref>, an audio system <b>10</b> includes a radio receiver and has an antenna input for connecting to an antenna <b>11</b> that is deployed to receive radio broadcast signals <b>12</b>. Radio broadcasts are selected and demodulated by the radio receiver and the resulting audio is amplified and then reproduced by speakers <b>13</b>.

In order to reproduce signals from an audio source <b>14</b> without needing an additional input to audio system <b>10</b>, a modulator <b>15</b> and a relay <b>16</b> are provided. <figref idref="DRAWINGS">FIG. 1</figref> shows relay <b>16</b> in a position to select signals from antenna <b>11</b> for presentation to the antenna input of audio system <b>10</b>. Modulator <b>15</b> is typically inactive while the antenna is selected, so that no local RF signal is being generated.

<figref idref="DRAWINGS">FIG. 2</figref> shows relay <b>16</b> in a position to select modulated signals from modulator <b>15</b> for presentation to the antenna input of audio system <b>10</b>. Thus, a local RF signal from modulator <b>15</b> and an antenna RF signal are both present at relay <b>16</b>. Due to the size and construction of relay <b>16</b>, a leakage or parasitic inductance <b>17</b> and a leakage or parasitic capacitance <b>18</b> are present between the antenna input terminal of relay <b>16</b> and the other terminals of relay <b>16</b>. The leakage tends to create cross-coupling such that 1) undesired radio broadcast signals from the antenna propagate to the radio receiver thereby causing interference with the local RF signal from modulator <b>15</b>, and 2) the local RF signal from modulator <b>15</b> propagates to antenna <b>11</b> creating unwanted RF emissions that may interfere with other nearby receivers.

<figref idref="DRAWINGS">FIG. 3</figref> shows a preferred implementation of the present invention for avoiding the foregoing problems. A modulator/switch unit <b>20</b> may be constructed as a separate module for connecting to a reception antenna, an accessory audio source, and a radio receiver. A relay <b>21</b> has its output connected to one input of a relay <b>22</b>. Relay <b>21</b> has one input connected to an antenna input connector of unit <b>20</b> and to a first node of a resonant isolator <b>23</b>. The other input of relay <b>21</b> is connected to a second node of resonant isolator <b>23</b>. Resonant isolator <b>23</b> has a ground node which is selectably connected to ground via a relay <b>24</b>. A relay driver <b>25</b> drives relays <b>21</b>, <b>22</b>, and <b>24</b> in common. To receive a radio broadcast from the antenna, the relays are positioned as shown so that relay <b>21</b> selects the antenna connector, relay <b>22</b> selects the output of relay <b>21</b> for connecting to the antenna input of the radio receiver, and relay <b>24</b> disconnects resonant isolator <b>23</b> from ground. Thus, resonant isolator <b>23</b> electrically floats so that it does not affect the antenna signals passing through modulator/switch unit <b>20</b>.

To reproduce an accessory audio signal, relays <b>21</b>, <b>22</b>, and <b>24</b> are switched to their opposite positions (from those shown in <figref idref="DRAWINGS">FIG. 3</figref>) so that relay <b>21</b> selects the second node of resonant isolator <b>23</b>, relay <b>22</b> selects the output of modulator <b>15</b>, and relay <b>24</b> connects the ground node of resonant isolator <b>23</b> to ground so that resonant isolator <b>23</b> becomes active. The cross-coupling of signals to and from the antenna connector via the leakage inductance and capacitance that would otherwise occur is negated by the action of resonant isolator circuit <b>23</b> which becomes connected across the open terminals of relay <b>21</b>. Due to the construction of resonant isolator <b>23</b>, any signal tending to propagate in either direction via the leakage inductance and capacitance is canceled out.

<figref idref="DRAWINGS">FIG. 4</figref> shows the relays and the resonant circuit in greater detail. A pair of double-pole, double-throw relays <b>30</b> and <b>31</b> are commonly controlled by relay driver <b>25</b> which is connected to magnetic actuators <b>32</b> and <b>33</b>, respectively. One half of relay <b>30</b> acts as the first relay for selectably connecting an antenna signal at a terminal <b>50</b> or a resonant isolator signal at a terminal <b>52</b> to a relay output terminal <b>51</b>. The other half of relay <b>30</b> is used to selectably connect isolator <b>23</b> to ground. The two halves of relay <b>31</b> are connected in series to act as a single-pole, double-throw relay for outputting either the antenna signal or the modulator signal to the socket for connecting to the antenna input of the radio receiver. The series connection decreases the effective capacitance of relay <b>31</b>, thereby increasing the impedance.

In the antenna mode shown in <figref idref="DRAWINGS">FIG. 4</figref>, antenna signals from a reception antenna are switched through the unit via an antenna socket <b>34</b>, through terminals <b>50</b> and <b>51</b> to relay <b>31</b>, and through terminals <b>60</b>, <b>61</b>, <b>62</b>, and <b>63</b> of relay <b>31</b> to a socket <b>35</b> having a cable connection to the antenna input of the radio receiver.

In the modulator mode shown in <figref idref="DRAWINGS">FIG. 5</figref>, terminals <b>50</b> and <b>51</b> of relay <b>30</b> are no longer connected and, except for leakage, the antenna signal is blocked by relay <b>30</b>. Since terminals <b>53</b> and <b>54</b> are switched together, a ground is now available for resonant isolator circuit <b>23</b>. The antenna signal is coupled to a first node <b>36</b> of isolator <b>23</b> through parallel resistors <b>37</b> and <b>38</b>. The equivalent resistance of resistors <b>37</b> and <b>38</b> is selected to closely approximate the characteristic impedance of the antenna cable connected to socket <b>34</b>. Isolator <b>23</b> includes a series resonant branch connected between first node <b>36</b> and a ground node <b>43</b>, and comprises parallel-connected capacitors <b>40</b> and <b>41</b> connected in series with an inductor <b>42</b>.

A second node <b>44</b> of isolator <b>23</b> provides a canceling output signal to terminal <b>52</b> of relay such that there is no interfering antenna signal present at terminal <b>51</b> for propagating to relay <b>31</b>. Conversely, any leakage signal of the modulator signal from relay <b>31</b> toward terminal <b>51</b> of relay <b>30</b> is canceled by isolator <b>23</b> before it can reach the antenna.

The canceling signal is generated at a second node <b>44</b> which is coupled to first node <b>36</b> by a load resistor <b>45</b>. Second node <b>45</b> is coupled to ground by a nulling branch comprised of a capacitor <b>46</b> connected in series with a parallel combination of an inductor <b>47</b> and a resistor <b>48</b>. In <figref idref="DRAWINGS">FIG. 5</figref>, the first series resonant branch and the nulling branch are independent resonant circuits which are independently adjustable by variable capacitors <b>41</b> and <b>46</b>, respectively. The first series resonant branch is tuned for a minimum impedance and then the nulling branch is tuned to provide a null balance across load resistor <b>45</b>. The resulting null signal which is coupled to terminals <b>52</b> and <b>51</b> of relay <b>30</b> is thus greatly attenuated (e.g., attenuation in the range of from 70 to 125 dB has been achieved). Since the resonant circuits are independently adjustable, a simple procedure can be followed wherein the first series resonant branch is adjusted to provide minimum impedance at (i.e., resonate at) the modulator frequency and then the nulling branch is adjusted to achieve a measured null at the second node. Subsequently, the first series resonant branch and the nulling branch are alternately re-adjusted to obtain further increases in attenuation.

The embodiment of <figref idref="DRAWINGS">FIGS. 4 and 5</figref> is especially well suited to providing isolation for a single radio channel at a time. When a single channel modulator is used, then manual tuning of the isolator circuit is all that is necessary. If the modulator is switchable between multiple radio channels, then provision can be made to automatically re-tune the resonant circuits.

Alternatively, an isolator circuit having a wider stopband and not needing any calibration or adjustment can be provided as shown in <figref idref="DRAWINGS">FIG. 6. A</figref> first node <b>70</b> is coupled to the antenna by a resistor <b>71</b> and to a second node <b>72</b> by a load resistor <b>73</b>. A first series resonant branch includes a capacitor <b>74</b> in series with an inductor <b>75</b>. A resistor <b>76</b> is connected in parallel with inductor <b>75</b> to “de-q” or reduce the frequency selectivity of the resonant circuit. The series resonant branch is coupled to ground through a very small resistance <b>78</b> (e.g., 2 ohms) which may be provided by the resistance of the relay itself. A nulling branch includes a capacitor <b>77</b> connected between second node <b>72</b> and resistor <b>78</b>. Load resistor <b>73</b> and capacitor <b>77</b> act as a phasor for generating the canceling signal at second node <b>72</b>. In this embodiment, a voltage divider comprising resistors <b>80</b> and <b>81</b> is added between second node <b>72</b> and the canceling output of the isolator circuit. The voltage divider especially assists in the attenuation of modulator signals passing through the leakage inductance and capacitance of second relay <b>31</b>.

<figref idref="DRAWINGS">FIG. 7</figref> illustrates the different modes of performance of the two different embodiments. Curve <b>85</b> shows the attenuation achieved using the more frequency-selective embodiment of <figref idref="DRAWINGS">FIGS. 4 and 5</figref>. Attenuation achieved using the circuit of <figref idref="DRAWINGS">FIG. 6</figref> is shown by curve <b>86</b>. Although less attenuation at the modulator frequency is obtained in curve <b>86</b>, better overall attenuation is achieved across the full FM band. Therefore, the same resonant isolator circuit can be used with any modulator frequency without requiring any adjustment or circuit calibrations. Furthermore, the attenuation is still great enough that the modulator can operate without interference even in areas where a radio broadcaster is present at the modulator frequency. Therefore, a less costly modulator without the capability of using multiple modulating frequencies can be used exclusively.